So I figured "don't start apache2" unless "remote-fs" has been
completed.

Better approach for this is

RequiresMountsFor=/srv

(or whatever paths your apache needs). This automatically adds
correct dependencies for the paths given. Again, not sure whether
this was available already in 12.3.

Looks like it is - the manpage mentions it, and I got no complaints
when I added it. I'll restart the box and see what happens.

This seems to have done the trick. I say 'seems' as I have had one
dependency failure during a startup, but this may have been related to
another change. Thanks for pointing me to "RequiresMountsFor", much
appreciated!